Infrared or thermal imaging systems typically use a plurality of thermal sensors to detect infrared radiation and produce an image capable of being visualized by the human eye. Thermal imaging systems typically detect thermal radiance differences between various objects in a scene and display these differences in thermal radiance as a visual image of the scene.
The basic components of a thermal imaging system generally include optics for collecting and focusing infrared radiation from a scene, an infrared detector or focal plane array (FPA) having a plurality of thermal sensors for converting infrared radiation to an electrical signal, and electronics for amplifying and processing the electrical signal into a visual display or for storage in an appropriate medium. A chopper is often necessary in thermal imaging systems to AC couple the detector to the scene. The electronic processing portion of the thermal imaging system will typically subtract the signal generated from the different chopper quadrants. An example of a thermal imaging system is described in U.S. Pat. No. 4,143,269, issued to McCormack, et al., entitled Ferroelectric Imaging System.
One type of thermal sensor includes a pyroelectric element formed from pyroelectric material that exhibits a state of electrical polarization and capacitance dependent upon temperature changes in response to incident infrared radiation. An infrared absorber and common electrode are disposed on one side of the pyroelectric elements. A sensor signal electrode may be disposed on the opposite side of each pyroelectric element. The infrared absorber and common electrode extend across the surface of the focal plane array and are attached to each of the pyroelectric elements. Each pyroelectric element generally has its own separate sensor signal electrode. Each infrared detector element or thermal sensor is defined in part by the infrared absorber and common electrode and the respective sensor signal electrode. The electrodes constitute capacitive plates and the pyroelectric element constitutes a dielectric disposed between the capacitive plates. A large biasing voltage across the detector decreases the capacitance of the detector. This, in turn, lowers the gain and increases the noise of the detector by up to three to five orders of magnitude.
These previously developed thermal sensors often require a chopper in order to obtain a reference signal. The chopper must be driven by a drive source and must be synchronized with the readings taken at the thermal sensors. This increases the complexity and cost of the thermal imaging system. These additional elements also require power, and therefore, may result in a system that may not be suitable for long-term battery powered operation.